Actually, no, Tomestones is correct. Next time you log in, take a careful look and you'll see that's how they are spelled. This is because they are not grave markers at all: they are Allagan data records. Stones containing tomes. They are highly valued because Allagan stuff in general is highly valued, and these happen to be books detailing how the Allagans lived, thought, and taught.
The current Tomestones that are in vogue are Tomestones recording selections of poetry (Poetics), laws and regulations (Law), and odd bits of info not covered under other Tomestone categories (Esoterics). Previous Tomestones no longer so highly valued included those recording philosophical musings (Philosophy), myths and legends (Mythology), and military matters (Soldiery).